Conventionally, a machine tool is known that includes a workpiece holding unit to hold a workpiece, a tool rest to hold a cutting tool for cutting the workpiece, a feeding unit to feed the cutting tool toward the workpiece in a predetermined feeding direction via relative movement between the workpiece holding unit and the tool rest, a repetitive movement unit to repetitively move the workpiece holding unit and the tool rest in a mutually relative manner by repeating the relative movement in the feeding direction at a first speed and a second speed that are mutually different, and a rotating unit to relatively rotate the workpiece and the cutting tool, the machine tool being capable of machining the workpiece via the relative rotation between the workpiece and the cutting tool and via the feeding of the cutting tool toward the workpiece with the repetitive movement. Examples of such machine tools are described in Japanese Patent No. 5139591 (see particularly paragraph 0039), and in Japanese Laid-Open Patent application No. H10-43901 (see paragraph 0019).
When one of these machine tool machines a workpiece, a cutting work position of a forward movement of a reciprocal vibration, which is one example of the repetitive movement, overlaps with a cutting work position of a backward movement of the reciprocal vibration. Thus, the cutting tool performs an “air cut,” in which the cutting tool simply moves without performing actual cutting during the backward movement because a part of the workpiece that is supposed to be cut in the backward movement has already been cut in the forward movement. Due to this, it is possible to machine a workpiece smoothly while sequentially separating a chip generated from the workpiece during the cutting work.
When the reciprocal vibration is performed such that the cutting work position of the forward movement overlaps the cutting work position of the backward movement, especially when the number of the reciprocal vibrations with respect to one rotation of a spindle that holds and rotates a workpiece is less than one, i.e., when the cutting tool vibrates once with respect to multiple rotations of the spindle, it is desirable to have a reciprocal vibration pattern that is capable of limiting degradation of machining efficiency.
For example, suppose that, in a reciprocal vibration pattern in which the reciprocal vibration is performed once with respect to four rotations of the spindle, which causes a tool edge of the cutting tool to trace a path in the form of a reciprocal vibration waveform illustrated in FIG. 6A, the path traced during a first rotation of the spindle, the path traced during a second rotation of the spindle, the path traced during a third rotation of the spindle, and the path traced during a fourth rotation of the spindle are indicated by a, b, c, and d, respectively. If the cutting tool performs a forward movement during the first and second rotations and performs a backward movement during the third and fourth rotations, the paths a to d with respect to the phase of the spindle can be illustrated as shown in FIG. 6B. In this case, an air cut occurs after an intersection point CT between the path c and the path d. This means that the air cut by the cutting tool occurs after the cutting work position of the backward movement overlaps the cutting work position of the forward movement in the third rotation of the spindle, and continues until the end of the fourth rotation of the spindle. Thus, the air cut is performed for more than one rotation of the spindle, causing degradation of cutting efficiency.
Thus, it is an object of the present invention to address the above problem of the conventional art by providing a machine tool that is capable of limiting degradation of machining efficiency when a cutting tool machines a workpiece by performing one repetitive movement with respect to multiple relative rotations between the workpiece and the cutting tool, and by providing a control apparatus of the machine tool.